Scenario: You've built a Referral Tracking solution within CRM. Custom entity "Referral Source" has a lookup to the User entity. If the Referral Type optionset == Sales, you want the view of the User lookup to be set to "Sales Users". If the Referral Type == Non-Sales, you want the view to be set to "Non-Sales Users".

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Currently, in order to filter the lookup appropriately using OOB design patterns I'd have to create two User lookups with the appropriate view hard-coded into each. Then via PBL hide/expose the "wrong" lookup depending on the situation. Further, to alleviate reporting issues, I'd need a third User lookup to pass the input into (via real-time workflow).

I know I can filter a lookup based on the input of another lookup; I'm want to filter the lookup based on *any* datatype (such as an optionset).